1. Field of the Invention
This invention relates to fluorescent phosphor materials. More particularly, it is concerned with non-stoichiometric modified strontium orthophosphate phosphors coactivated with tin and manganese.
2. Discussion of the Prior Art
Fluorescent phosphor materials employing a matrix of calcium or strontium orthophosphate have been the subject of some study.
For example, Kroger in Physica, 15:801 (1949) discusses the fluorescence of unmodified calcium or strontium orthophosphate phosphors coactivated with tin and manganese or with cerium and manganese.
Butler in J. Electrochem. Soc., 100:250 (1953) describes alkaline earth orthophosphate phosphors activated with tin or with tin and manganese. The tin-manganese coactivated phosphors disclosed by Butler employ a calcium orthophosphate host matrix modified by the incorporation of some strontium.
Koelmans et al. in J. Electrochem. Soc., 104:442 (1957) discuss tin activated strontium orthophosphate phosphors modified by the incorporation of aluminum, cadmium, calcium, magnesium, or zinc.
Sarver et al. in J. Electrochem. Soc., 108:1103 (1961) discuss phase equilibrium relationships and luminescence of tin activated strontium orthophosphate phosphors modified with calcium, magnesium, or zinc.
Calcium or strontium orthophosphate phosphors which are activated by tin alone possess short fluorescent decay lifetimes. While short decay lifetime is a useful property of phosphors in some applications, it may lead to undesirable results in some instances as, for example, the flicker of fluorescent lamps which incorporate such phosphors. This flicker is a particular problem when fluorescent lamps are energized by alternating current of lower frequencies such as the 50 Hz. frequency common in European countries.
When additional ions are incorporated into tin activated calcium or strontium orthophosphate hosts, there is often a shift in the color of light emitted by the resultant phosphor upon excitation. This color shift can be undesirable when the choice of a phosphor for a particular application is based on the desired fluorescence color of tin.